THE ONE SIDED FOLD. 291 



complete plication of the formations at that end of the range. This ridge 

 may extend westward at a greater depth along the whole length of the 

 range, and be the unyielding mass whose resistance caused the sharper flex- 

 ure along its northern edge. 



I also differ from the views advanced by some geologists with regard 

 to the structure of the Rocky Mountain region, or the Park province, as 

 they designate it. One considers the type structure of this region, as repre- 

 sented by the Colorado and Park Ranges, to be the same as that of the Uinta, 

 viz, that the sedimentary strata formerly arched over them, and that the 

 uplift was that of a broad platform raised by vertical movement, having a 

 fault or monoclinal fold along either edge. Another, in speaking of the 

 whole Cordilleran system east of the Sierra Nevada, says it has no plication 

 properly speaking, as expressed by the folds of the Appalachian, although 

 he admits that some regions show a certain amount of flexure, including in 

 them probably the Basin and Park provinces. In regard to these provinces 

 he says that these flexures are not, so far as can be discerned, associated with 

 the building of the existing mountains in such a manner as to justify the 

 inference that the flexing and the rearing of the ranges are correlatively 

 associated ; that the flexures were in the main older than the mountains, and 

 that the mountains were blocked out by faults from a platform which had 1 

 been plicated long before, and after the irregularities due to such pre-ex- 

 isting flexures had been nearly obliterated by erosion. He says further that 

 the amount of bending caused by the uplifting of the ranges is just enough 

 to give the range its general profile and seldom anything more. 



I have already shown, in Chapter II, my reasons for considering that 

 the main Archean masses of the Rocky Mountains, as represented by the 

 Colorado and Park Ranges, were never submerged, and that therefore the 

 sedimentary strata could not have arched over them as they did over the 

 Uinta Range; also, that the Mosquito Range, which might be considered 

 at first sight an exception, is not geologically part of the Park Range, but an 

 uplift of later formation, contemporaneous with and of analogous formation to 

 the so-called " Hog-back" ridges on the east flanks of the Rocky Mountains, 

 though of more complicated structure, owing partly to its eruptive masses and 



